1. Field of the Invention
This invention relates to a rare gas discharge fluorescent lamp for use with an information device such as a facsimile, a copying machine or an image reader wherein fluorescent substance is excited to emit light by ultraviolet rays generated by rare gas discharge.
2. Description of the Prior Art
In recent years, the performances of information terminal devices such as a facsimile, a copying machine and an image reader have been improved together with advancement of the information-oriented society, and the market of such information devices is rapidly expanding. In developing information devices of a higher performance, a light source unit for use with such information devices is required to have a higher performance as a key device thereof. Conventionally, halogen lamps and fluorescent lamps have been employed frequently as lamps for use with such light source units. However, since halogen lamps are comparatively low in efficiency, fluorescent lamps which are higher in efficiency are used principally in recent years.
However, while a fluorescent lamp is high in efficiency, it has a problem that characteristics thereof such as the fact that an optical output characteristic varies in accordance with a temperature since discharge from mercury vapor is utilized for emission of light. Therefore, when a fluorescent substance is used, either the temperature range in use is limited, or a heater is provided on a wall of a tube of the lamp in order to control the temperature of the lamp. However, development of fluorescent lamps having stabilized characteristics are demanded eagerly for diversification of locations for use and for improvement in performance of devices. From such background, a rare gas discharge fluorescent lamp which makes use of emission of light based on rare gas discharge and is free from a change in temperature characteristic is being developed as a light source for an information device.
FIGS. 19 and 20 show an exemplary conventional rare gas discharge fluorescent lamp devices which is disclosed, for example, in Japanese Patent Laid-Open No. 63-58752, and wherein FIG. 19 is a diagrammatic representation showing a longitudinal section of a rare gas discharge fluorescent lamp and an entire construction of the device, and FIG. 20 is a cross sectional view of the lamp. Referring to FIGS. 19 and 20, the rare gas discharge fluorescent lamp of the device shown includes a bulb 1 in the form of an elongated hollow rod or tube which may be made of quartz or hard or soft glass. A fluorescent coating 2 is formed on an inner face of the bulb 1, and rare gas consisting at least one of xenon, krypton, argon, neon and helium gas is enclosed in the bulb 1. A pair of inner electrodes 3a and 3b having polarities opposite to each other are located at the opposite longitudinal end portions within the bulb 1. The inner electrodes 3a and 3b are individually connected to a pair of lead wires 4 which extend in an airtight condition through the opposite end walls of the bulb 1. An outer electrode 5 in the form of a belt is provided on an outer face of a side wall of the bulb 1 and extends in parallel to the axis of the bulb 1.
The inner electrodes 3a and 3b are connected by way of the lead wires 4 to a high frequency invertor 6 serving as a high frequency power generating device, and the high frequency invertor 6 is connected to a dc power source 7. The outer electrode 5 is connected to the high frequency invertor 6 such that it may have the same polarity as the inner electrode 3a.
Operation of the rare gas discharge fluorescent lamp device is described subsequently. With the rare gas discharge fluorescent lamp device having such a construction as described above, if a high frequency power is applied across the inner electrodes 3a and 3b by way of the high frequency invertor 6, then glow discharge will take place between the inner electrodes 3a and 3b. The glow discharge will excite the rare gas within the bulb 1 so that the rare gas will emit peculiar ultraviolet rays therefrom. The ultraviolet rays will excite the fluorescent coating 2 formed on the inner face of the bulb 1. Consequently, visible rays of light are emitted from the fluorescent coating 2 and discharged to the outside of the bulb 1.
Another rare gas discharge fluorescent lamp is disclosed, for example, in Japanese Patent Laid-Open No. 63-248050. The lamp employs such a hot cathode electrode as disclosed, for example, in Japanese Patent Publication No. 63-29931 in order to eliminate the drawback of a cold cathode rare gas discharge lamp that the starting voltage is comparatively high. The rare gas discharge fluorescent lamp can provide a comparatively high output power because its power load can be increased. However, it can attain only a considerably low efficiency and optical output as compared with a fluorescent lamp based on mercury vapor.
In summary, conventional rare gas discharge fluorescent lamps cannot attain a sufficiently high brightness or efficiency as compared with fluorescent lamps employing mercury vapor because fluorescent substance is excited to emit light by ultraviolet rays generated by rare gas discharge.